Genetic Crosses

1. Genetic Crosses Do Now: In rats, black fur color is dominant to white fur color. If a black rat has one copy of the black fur allele (B) and one copy of the white fur allele (b), what % of the gametes it produces will carry the white fur allele?

2. Phenotype & Genotype • Phenotype is the characteristic an organism has because of the alleles it has. • Examples: brown fur, white flowers, dwarfism, blue eyes, etc… • Genotype is the different alleles an organism has, represented by letters • Examples: Bb, pp, Dd, bb, etc… • What is the phenotype of the rat in the do now? • What is it’s genotype?

3. Heterozygous & Homozygous • The genotype of an organism can be heterozygous or homozygous • Heterozygous individuals have two different alleles for a gene – like the rat in the do now. (Hetero = different) • Example: Bb = one brown eye allele, one blue eye allele • Homozygous individuals have two of the same allele. (Homo = same) • Example: BB, bb, PP, pp, etc…

4. Dominant & Recessive alleles • Since a homozygous individual has two identical alleles, their phenotype will be whatever those alleles are • Genotype = BB; Phenotype = brown eyes • Genotype = bb; Phenotype = blue eyes • A heterozygous individual will have the dominant trait. • A dominant allele is on that masks or hides the presence of a recessive one. • Genotype = Bb; Phenotype = brown eyes • Brown eye allele (B) is dominant to blue eye allele (b)

5. Practice • In cows, having spots (S) is dominant to no spots (s) • What is the phenotype of a cow that has spots? • What is the genotype of a heterozygous cow? • Will a heterozygous cow have spots? • What is the genotype of a cow with no spots? • If a cow has spots, what 2 genotypes might it have?

6. More Practice • In pea plants, yellow seed color (Y) is dominant to green seed color (y). • What is the genotype of a plant with green seeds? • What is the phenotype of a Yy plant? • Is a YY plant homozygous or heterozygous? • Is a yy plant homozygous or heterozygous? • If a YY plant self-fertilizes, what color will the seeds of its offspring be?

7. Punnett Square • A diagram that helps predict the outcome of a genetic cross. • The offspring produced by a genetic cross are described in two ways: • Phenotype ratio: the ratio of one phenotype to the other. • Example: 2 black rats, 2 white rats = 1:1 phenotype ratio • Genotype ratio: the ratio of all of the different genotypes of the offspring • Example: 1 BB, 2 Bb, 1 bb = 1:2:1 genotype ratio

8. Genetic Crosses Y Y Y y y y x Y Y Y y y y Y Y Y y Y y y y 3:1 1:2:1

9. Practice • The example problems on your worksheet will let you know if you’ve picked up the skills you need to be a future geneticist! • Work on them now, so that I can help before you go. • DON’T FORGET THE MITOSIS LAB!!!